Specific association of SLC6A4, but not BDNF, methylation with uncinate fasciculus integrity in drug-naïve major depressive disorder

Introduction

Major Depressive Disorder (MDD) is associated with white matter (WM) structural abnormalities and distinct epigenetic modifications. As current diagnostic frameworks rely primarily on descriptive symptomatology, identifying objective biomarkers is essential to unravel the disorder’s underlying biological heterogeneity. The serotonin transporter (SLC6A4) and the Brain-Derived Neurotrophic Factor (BDNF) genes are critical regulatory targets deeply implicated in MDD vulnerability, emotional processing, and neuroplasticity. However, the specificity of the relationship between these epigenetic markers, tract-specific WM integrity, and clinical symptoms in drug-naïve MDD remains unclear. Investigating this epigenetic-neural axis is crucial for discovering objective, biologically based diagnostic biomarkers.

Methods

This study investigated 31 drug-naïve MDD patients (DNM) and 62 healthy controls (HC). Diffusion Tensor Imaging (DTI) data were analyzed using TRACULA (TRacks Constrained by an Anatomical Atlas), an automated probabilistic tractography tool, to obtain Fractional Anisotropy (FA) values for 18 major WM tracts. Concurrently, promoter methylation levels of SLC6A4 (quantified at five distinct CpG sites) and BDNF (positions 1-4) were measured using genomic DNA extracted from peripheral whole blood. Clinical symptom severity was assessed via Hamilton Depression Rating Scale (HDRS), Hamilton Anxiety Scale (HAS), and the Suicide Severity Index (SSI). To explore the epigenetic-neural relationships, partial correlation analyses were conducted, controlling for age, sex, total intracranial volume (TICV), and education years.

Results

The DNM group showed significantly higher HDRS scores and elevated SLC6A4 methylation at CpG2 and CpG3 compared to the HC group. Conversely, there were no significant inter-group differences in BDNF methylation. DTI analysis revealed significantly decreased FA in the Forceps Major and Left Inferior Longitudinal Fasciculus in the DNM group. Within the DNM group, SLC6A4 CpG4 methylation showed a significant negative correlation with the FA of the Left Uncinate Fasciculus (L-UF) (r = -0.560, p = 0.002), which survived Bonferroni correction (p < 0.00278). Furthermore, SLC6A4 CpG5 methylation negatively correlated with both HDRS (r = -0.423, p = 0.028) and SSI (r = -0.500, p = 0.008). In contrast, BDNF methylation showed no significant correlations with any of the 18 TRACULA-derived FA values or clinical scales (HDRS, SSI, HAS) in the DNM group.

Conclusions

This comprehensive TRACULA-based study reveals a highly specific, multidimensional pathophysiological link in drug-naïve MDD. The lack of structural and clinical associations with BDNF methylation highlights the specificity of the serotonergic epigenetic-neural axis. Specifically, elevated SLC6A4 methylation at CpG4 is uniquely associated with microstructural compromise in the L-UF, a critical frontolimbic tract essential for emotional regulation. These findings suggest that SLC6A4 site-specific methylation acts as a distinct epigenetic vulnerability marker. This distinct epigenetic-neural link offers a framework for biologically based patient subtyping, moving beyond descriptive symptomatology to support the development of circuit-targeted precision treatments. Integrating multi-modal epigenetic and neuroimaging data facilitates the identification of objective, cross-modal biomarkers that may pave the way for the early diagnosis and personalized management of depression.

References

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